1. Field Of The Invention
The present invention relates to scanners and, more particularly to scanners having single or multiple arrays wherein an electronic image of an original document is generated from the arrays. The electronic image is processed to reconstruct a copy of an original document.
2. Description of The Prior Art
The use of scanners for generating electronic images of an original document is well known in the prior art. Prior art scanners may be classified into two groups. The so called low resolution scanners and the so called high resolution scanners.
In the low resolution scanners a straight line of information of an original document is projected onto a linear diode array. The linear array outputs a video signal representative of the line of information on the original. With the low resolution scanners only one linear array is used to reproduce a continuous line of data on an original document.
With the high resolution scanners two or more linear arrays, such as diode arrays, are used for generating the video signal for a straight line of data on an original document. In order to achieve the high resolution each character on a straight line of an original document is divided or partitioned into a plurality of Picture Elements (PEL). A typical PEL size is within the range of several microns. Each PEL is projected onto a diode, or other photosensitive element in the arrays. As such, a relatively large number of diodes are needed to reproduce a video signal of a straight line of data on an original document.
It would be desirable to have the large number of diodes or other sensing elements required for the high resolution scanner packaged in a linear array. However, due to limitations imposed by the physical size of the scanner, the mechanical configuration of the arrays and, more important due, to limitations imposed by the solid state or semi-conductor technology (that is the manufacturers of the arrays) the number of diodes positioned linearly on a substrate (that is the length of an array) is fewer than the number of diodes necessary to reproduce a high resolution copy of a continuous line on an original document.
The aforementioned imposed limitations are overcome by projecting one continuous line of a document onto a plurality of linear arrays. With respect to a straight line of data running from left to right on a page, the arrays are positioned in an over-lapping offsetting fashion. Stated another way, in order to generate a video signal representative of a straight line of data extending from a left margin to a right margin of an original document, a first linear diode array is positioned so as to cover a portion of the line. A second linear diode array is positioned so that the beginning of the diodes in the second linear arrays overlap with the diode of the first linear array. Likewise, a third and N linear arrays are arranged in a fashion similar to that described for for the first and second arrays. In other words, a plurality of arrays are arranged to cover a continuous line of data on the original document. Usually the arrays are offset with respect to one another in the direction of scan. Also the arrays are overlapped in a direction parallel to a line on the original document. By way of prior art example, U.S. Pat. Nos. 4,005,285 and 4,092,632 give a more detailed description of a multiple array scanner.
When video data is reproduced by the aforementioned multiple arrays scanner, several types of dimensional variations or errors are associated with the video data.
One type of error which is associated with the prior art multiple array scanners is the so called abutment error. The abutment error usually occurs at the juntion point or crossover point of successive arrays. The abutment error generally manifests itself in two forms. In one form the video information at the crosspoint is redundant. The redundant information arises because for some finite period of time the overlapped element of the arrays are reading the same information. The other form by which the alignment error manifests itself is that of separation. This means that the video output from succeeding arrays are separated by a gap.
Another type of error which is usually associated with multiple array scanners is the so called misalignment error. As was stated previously, with a multiple array scanner each line of data on an original document is reproduced by the composite output from a plurality of arrays. Due to misalignment between the arrays or misalignment between the original document and the arrays the output from each array is offset relatively. Usually the offset is in the direction of scan.
A third type of error which is associated with the multiple array scanners is the skew error. With the skew error the video data outputted from the array is rotated relative to a center line or reference point taken horizontally across the document plates. The skew may be positive or negative depending on its position relative to the center line.
Of all the above described dimensional defects associated with multiple array scanners, only one (the so called skew defect) is associated with the single element array scanners. The invention described hereinafter can be used with single element array scanners to correct the skew associated therewith.
Although the dimensional defects (such as skew, abutment, and alignment) are well known to the prior art, only the abutment and alignment defects are addressed. To date no prior art could be found in which the skew defect is addressed.
One method used in the prior art to effectuate alignment is that each of the arrays in the direction of scan, is offset a predetermined distance from a start of scan line. The individual distance for each array is determined and stored in a series of offset counters. Each of the counters are associated or dedicated to each array. The offset counters serve, at the start of a scan, to delay activation of the arrays until the distance associated therewith is traversed.
In order to effectuate abutment between data outputted from the different arrays scanning a straight line of an original document, a vernier scale is fabricated on one of the arrays. The vernier scale is located at one end of the array. The vernier array is positioned relative to the non-vernier array so that the vernier scale is located at the overlapping zone of the arrays. The vernier scale is achieved by placing the photosensitive elements of the vernier section at a center-to-center distance which is shorter than the center-to-center distance of the photosensitive elements in the non-vernier section of the arrays. The reduction in center-to-center distance between elements in the vernier section of the array provides at least one point where successive arrays are in alignment. The point is called the crossover point. The crossover point is determined by microscopic examination of the arrays. By way of example, a more detailed description of the prior art method of correcting abutment and alignment is given in U.S. Pat. No. 4,092,632.
Although the prior art approach to correcting defects associated with multiple array scanners appears to perform satisfactorily, it is lacking in some respect. For example, the prior art does not address all of the dimensional defects associated with multiple array scanners. More particularly, the prior art does not correct the defect in a reproduced document due to skew.
Moreover, in order to correct for abutment at least one of the arrays has to be custom made in order to have the reduced center-to-center distance needed in the vernier portion of the array. As is well known to those skilled in the art, custom built electronic components tend to be much more expensive than off the shelf components. Expensive components tend to increase the overall cost of the system.
Another problem associated with the prior art is that the correction scheme is a static one. However, the forces or factors which influence the above dimensional variations are due to mechanical inaccuracies. These inaccuracies occur during initial set up, and mechanical instability due to time, temperature and mechanical shock. Since the factors are dynamic it would be expedient and more efficient to have a dynamic method to correct the dimensional variation associated with multiple array scanners.